Mast-mounted vibration isolators are well-known in the art for canceling or substantially reducing vibratory forces active on a helicopter rotor. While most such devices are referred to as "vibration absorbers", this may be viewed as a misnomer inasmuch as these devices typically isolate the energy produced by cyclic in-plane and out-of-plane loads rather than absorb the energy as the name implies. Such devices typically include: a hub attachment fitting for mounting to the main rotor hub such that the isolator is rotated in a plane parallel to the main rotor disc, and a spring-mass system mounted to and rotating with the hub member. The spring-mass system is tuned in the non-rotating condition to a frequency equal to N*rotor RPM (e.g., 4P for a four-bladed rotor) at normal operating speed, so that in the rotating condition it will respond to both N+1 and N-1 frequency vibrations (3P and 5P).
Vincent el al. U.S. Pat. Nos. 4,145,936 and 4,225,287 disclose mast-mounted vibration isolators having a plurality of resilient arms (i.e., springs) extending in a spaced-apart spiral pattern between a hub attachment fitting and a ring-shaped mass. More specifically, the hub attachment fitting is typically a solid block mounting to and rotating with the helicopter main rotor hub. At least three pairs of spiral springs (3 upper and 3 lower) are equiangularly arranged with respect to both the hub attachment fitting and the ring-shaped mass so as to produce substantially symmetric spring stiffness in an in-plane direction. Each spring is comprised of unidirectional fiberglass so as to provide low in-plane bending stiffness and superior fatigue properties. The ring-shaped mass is typically comprised of three segments which include a central ring and a pair of ring-shaped plates mounted in combination therewith so as to produce a substantially C-shaped cross-section. As such, the ring-shaped plates stiffen the central ring to obviate distortion and the adverse consequences of higher harmonic dynamic resonances.
While the teachings disclosed in the '936 and '287 patents provide a baseline for design and development, the isolators described therein are disadvantageous for various reasons. Firstly, the mounting of such isolators to the main rotor hub produces difficulties with respect to periodic inspection and maintenance of the rotor hub attachment, i.e., the structural connection between the main rotor hub and the main rotor drive shaft. Insofar as such structural connection is commonly checked and/or tightened following each flight, it is necessary to remove such isolators to perform inspection and maintenance. As such, it will be appreciated that the assembly/disassembly of the isolator with each flight adversely impacts the direct maintenance costs of the rotorcraft. Secondly, the configuration of such isolators does not accommodate various systems which extend up and/or through the main rotor shaft. For example, the distributor and wiring harnesses of a rotor blade de-ice system must be routed beneath or around the isolator rather than being centrally located above the main rotor hub or the isolator itself. Thirdly, the relatively large profile area and blunt configuration of such isolators produces high aerodynamic drag which adversely impacts rotorcraft performance and fuel consumption. Finally, the turbulent airflow produced by such isolators can adversely affect the handling qualities of the rotorcraft. For example, such turbulent airflow may, in certain flight regimes, impose undesirable vibratory loads on the tail pylon and produce objectionable vibration.
With respect to the latter issues associated with aerodynamic drag and handling qualities, it is known in the art to employ aerodynamic fairings over such vibration isolators to reduce aerodynamic drag penalties, reduce airflow turbulence and alter the course of the airflow downstream of the isolator. While such aerodynamic fairings substantially improve the performance and handling qualities of the rotorcraft, such fairings are parasitic in weight and cost.
A need, therefore, exists for a vibration isolator which eliminates the parasitic weight of a conventional rotor head fairing, minimizes aerodynamic drag, improves rotorcraft handling qualities, facilitates periodic inspection and maintenance of the main rotor hub attachment, and accommodates the mounting of various hub-mounted rotor systems.